89 European Journal of Pharmacology, 43 ( 1977 ) 89--90 © Elsevier/North-Holland Biomedical Press Rapid communication EFFECTS OF METHIONINE-ENKEPHALIN ON PROLACTIN RELEASE AND CATECHOL- AMINE LEVELS AND TURNOVER IN THE MEDIAN EMINENCE LOUISE FERLAND**, KJELL FUXE*, PETER ENEROTH**, JAN-AKE GUSTAFSSON*** and PAUL SKETT*** *Department of Histology, and ***Department of Chemistry, Karolinska Instituter, S-104 01 Stockholm 60, Sweden, and **Hormone Laboratory, Karolinska Hospital, S-104 01 Stockholm 60, Sweden Received 7 March 1977, accepted 8 March 1977 Hughes et al. have recently isolated and described the structure of a pentapeptide from pig brain with potent morphine-like ac- ticity. This pentapeptide is named methionine- enkephalin (met-enkephalin) and is struc- turally identical to the 61--65 sequence of ovine ~-lipotropin (~-LPH). Recent findings indicate the presence of enkephalin-like im- munoreactive material in nerve terminals of the medial basal hypothalamus of rat (Elde et al., 1976). Furthermore, Lien et al. (1976) and Labrie (personal communication, 1976) have found that met-enkephalin increased prolactin release in the rat as has been ob- served after morphine treatment. In the present study, the effects of met- enkephalin on catecholamine (CA) levels and on CA disappearance after tyrosine hydroxyl- ase inhibition have been studied as well as the effects on prolactin serum levels. Met-enkephalin (Peninsula Laboratories, San Carlos, Calif., U.S.A.) was infused (60 pg in 20 pl of sterile physiological saline) during 1 h into the lateral ventricle of adult male rats under fluothane-air anesthesia. Rats were de- capitated at the end of the infusion. The brains were immediately dissected out and the blood was centrifuged and kept at --20°C until radioimmunoassayed for prolactin (NIH kit). By means of a quantititave microfluorimetrical application of the Falck--Hillarp technique *Fellow of the Medical Research Council of Canada. (LBfstrSm et al., 1976) the CA stores in the median eminence were measured. The sub- ependymal layer (SEL). contains mainly noradrenaline (NA) nerve terminals and the lateral palisade zone (LPZ) mainly dopamine (DA) nerve terminals (LSfstr6m et al., 1976). In the medial palisade zone (MPZ) a mixture of DA and NA nerve terminals is present, where DA nerve terminals predominate (50-- 75% of total CA nerve terminals) (LSfstrSm et al., 1976). Changes in CA turnover were evaluated by studying the depletion of CA stores following treatment with a tyrosine hydroxylase inhibitor (~-methyltyrosine methylester, H44/68). This decline is, by and large, exponential in the median eminence (LSfstrSm et al., 1976). H44/68 was injected i.p. (250 mg/kg in physiological saline) at the same time as the infusion of met-enkephalin was started. After infusion of met-enkephalin alone (60 pg/1 h), the CA stores in the MPZ of the median eminence were increased by about 35% whereas the CA stores in the SEL were reduced by 28% (fig. 1). A trend towards an increase of CA stores in the LPZ was also noted. A marked increase (400%) in pro- lactin serun~ levels occurred at the same time. In experiments where H44/68 was used in addition, a significant reduction of the H44/ 68-induced CA disappearance was observed in both the MPZ and LPZ, whereas there were no significant changes in the H44/68-induced